enc28j60: Fix sporadic packet loss (corrected again)

Packet data read from the RX buffer the when the RSV is at the end of the RX
buffer does not warp around. This causes packet loss, as the actual data is
never read. Fix this by calculating the right packet data location.

Thanks to Shachar Shemesh for suggesting the fix.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Acked-by: Claudio Lanconelli <lanconelli.claudio@eptar.com>
Signed-off-by: David S. Miller <davem@davemloft.net>